The roots of Angelica gigas (Angelicae Gigantis Radix, AGR) have been used as an herbal medicine for blood stasis and inflammation in Korea. The main ingredients of AGR are decursin (D) and decursinol angelate (DA) belonging to coumarins. However, D and DA present a challenge for separation due to their structural isomerism. The incomplete separation interferes with quantification of D and DA leading to difficulties in quality control of AGR. From these backgrounds, high-performance liquid chromatography (HPLC) method for simultaneous determination of D and DA was established with a C30 column. D and DA were well-separated by a C30 column, resulting in a 2.44 resolution. In addition, the neuroprotective and anti-inflammatory effects of D and DA were compared in HT22 and Raw264.7 cells, respectively. D and DA showed different neuroprotective activities against glutamate-induced HT22 cell death; D showed potent activity, whereas DA was negligible. In contrast, the anti-inflammatory activities of D and DA were similar in lipopolysaccharide-stimulated Raw264.7 cells. This study indicates that separating D and DA is vital for quality control of AGR and pharmaceutical applications based on the differences in the pharmacological properties of each compound.